In the context of the present application, the term ‘object’ or ‘object to be imaged’ covers a turbid medium. A turbid medium is to be understood to mean a substance consisting of a material having a high light scattering coefficient, such as for example an intralipid solution or biological tissue. Further, light is to be understood to mean electromagnetic radiation of a wavelength in the range from 400 nm to 1400 nm. The term “optical properties” covers the reduced scattering coefficient μ′s and the absorption coefficient μa. Furthermore, “matching optical properties” is to be understood as having a similar reduced scattering coefficient μ′s and a similar absorption coefficient μa.
In recent years, several methods and devices for examining turbid media, e.g. female breast tissue, have been developed. In particular, new devices for detection and analysis of breast cancer have been developed and existing technologies have been improved. Breast cancer is one of the most occurring types of cancer: in 2002, for example, more than 1.1 million women were diagnosed and over 410.000 women died of breast cancer worldwide. Several types of devices for imaging the interior of a turbid medium by use of light have been developed. Examples for such devices are mammography devices and devices for examining other parts of human or animal bodies. A prominent example for a method for imaging the interior of a turbid medium is Diffuse Optical Tomography (DOT). In particular, such devices are intended for the in vivo localization of inhomogeneities in breast tissue of a part of a breast of a female human body. A malignant tumor is an example for such an inhomogeneity. The devices are intended to detect such inhomogeneities when they are still small, so that for example carcinoma can be detected at an early stage. A particular advantage of such devices is that the patient does not have to be exposed to the risks of examination by means of ionizing radiation, as e.g. X-rays.
WO 00/56206 A1 discloses a device for imaging the interior of a turbid medium by using a light source to irradiate the turbid medium and photodetectors for measuring a part of the light transported through the turbid medium. A control unit is provided for reconstructing an image of the interior of the turbid medium on the basis of the measured intensities. The disclosed device is particularly adapted for examining female breasts. In order to allow the examination of the turbid medium, the device is provided with a receptacle as a receiving portion enclosing a measuring volume and arranged to receive the turbid medium. Due to different sizes of the turbid media to be examined, the size of the receptacle for receiving the turbid medium does not perfectly match the size of the turbid medium, i.e. a space remains between the receptacle and the turbid medium.
The light used for examining the turbid medium has to be transmitted from the light source to the turbid medium and from the turbid medium to the photodetectors. A number of light paths coupling to the light source and a number of light paths coupling to photodetectors may be distributed across the wall bounding the measurement volume, for instance, ends of optical fibers acting as light guides are connected to the wall of the measurement volume. In a DOT measurement, the light source subsequently irradiates the turbid medium from different directions and the photodetectors measure a part of the light transmitted through the turbid medium. A plurality of such measurements are performed with the light directed to the turbid medium from different directions and, based on the results of the measurements, the control unit reconstructs the image of the examined turbid medium.
An optical matching medium for conducting optical energy generated by a light source at least from the light source to a turbid medium to be irradiated with at least a part of the optical energy generated by the light source is known from U.S. Pat. No. 5,907,406. The known optical matching medium can be used for imaging an interior of a turbid medium, such as biological tissue, using diffuse optical tomography. In medical diagnostics the matching medium may be used, for instance, for imaging an interior of a female breast. In that case, at least a part of the turbid medium, in this case a female breast, may be accommodated in the measurement volume.
In U.S. Pat. No. 5,907,406 the measurement volume is bounded by a cuplike wall portion. However, this is not always necessary. Inside the measurement volume, the part of the turbid medium under investigation is surrounded by the matching medium, so that the space inside the measurement volume not filled by the object to be imaged in general or the turbid medium in particular is filled with the matching medium. The matching medium is chosen such that the optical parameters of the matching medium, such as the absorption and scattering coefficients, are substantially identical to the corresponding optical parameters of the turbid medium. In this way, image artifacts resulting from optical boundary effects that occur when light is coupled into and out of the turbid medium can be reduced. Furthermore, use of the matching medium prevents the occurrence of an optical short-circuit in the receiving volume around the turbid medium. An optical short-circuit occurs when light is detected that has propagated along a path inside the receiving volume but outside the turbid medium and, as a consequence, has not been sufficiently scattered and attenuated. In that case the intensity of the insufficiently scattered and attenuated detected light may dwarf the intensity of detected light that has been scattered and attenuated through passage through the turbid medium.
For certain types of imaging processes, for instance, certain types of imaging an interior of a female breast, the object to be imaged is compressed between two surfaces. The question then is how to combine compression of the object to be imaged with the use of a matching medium. Several approaches have been proposed, see, for instance, Time-Domain Optical Mammography SoftScan: Initial Results, X. Intes, Acad. Radiol. 2005, 12:934-947 and Diffuse Optical Tomography of Breast Cancer during Neoadjuvant Chemotherapy: A Case Study with Comparison to MRI, R. Choe et al. MedPhys 32 (4) 2005, 1128-1139.
The latter approach has the drawback that it uses a movable plate that is submerged in the matching medium. This limits the light sources that can be used because they are sub much too. It also requires large amounts of matching medium.